Recently, it is advanced to increase a strength of a structural material for a vehicle body for the purpose of reducing a weight of an automotive vehicle body. Especially, it is attempted to use high-strength hot rolled steel sheets, which are also beneficial in view of the cost.
However, as the strength of the steel sheet is increased, the ductility generally lowers, and cracks, wrinkles and the like are apt to be easily created. Also, a spring back quantity is increased after the press forming to deteriorate the shape fixability and thereby lower the shape precision and hence there are caused problems such as dimension error and the like.
Particularly, the wheel disk is subjected to a baking after the press forming and then assembled into an automobile. Since this part is an important safety part relating to a running safeness of the automobile, it is required to have a severer durability to fatigue. In such a part, therefore, fatigue durability after the part forming-painting is also very important.
As the conventionally known high-strength hot rolled steel sheet, there are most generally so-called low-alloy and high-strength steel sheets (HSLA steel) obtained by adding not more than about 0.2 mass % of Nb or Ti, V and the like to a low-C steel.
That steel sheet has an advantage that the production can be conducted relatively easily and cheaply, but has a problem that the yield ratio is high and hence the shape fixability after the forming is poor.
In JP-A-60-181230 are proposed hot rolled steel sheets attempted by increasing the strength through a dual phase microstructure of ferrite and bainite. The ductility can be improved by such a microstructure form. In the microstructure having a secondary phase composed mainly of bainite, however, since the yield ratio is high, there is a problem that the shape fixability after the forming is poor likewise the HSLA steel.
In JP-B-56-54371 and JP-B-61-11291 are proposed steel sheets having a low yield stress and a good balance between strength and elongation in which a primary phase is ferrite and a secondary phase is a hard martensite phase.
However, such steel sheets indicate reasonable fatigue properties as a matrix, but when they are applied to the wheel disk for the automobile, there is left a problem that a higher fatigue durability is not obtained after the forming into parts.
It could therefore be advantageous to provide a high-strength hot rolled steel sheet having an excellent shape fixability as hot-rolled, an excellent fatigue durability after the forming and excellent weldability and phos-phatability and a tensile strength of not less than 590 MPa level as a first object.
It could also be helpful to provide a method of advantageously producing the above high-strength hot rolled steel sheet.